Liquid core waveguide or light guiding structures can be beneficial for guiding ultraviolet (UV) radiation, e.g., due to the low UV absorbance properties of some liquids, such as purified water. The general availability of water allows for the fabrication of relatively inexpensive light guides for UV radiation that can be readily adopted for use in industry. Combined with a light guide enclosure formed of a fluoropolymer having low UV absorbance and other beneficial properties (e.g., chemical inertness, low biological contamination), the benefits of thin light guiding UV layers can be easily appreciated.
These light guiding structures, or so-called liquid core waveguides or flow cells, have been developed for optical spectroscopy applications in the ultraviolet, visible, and infrared regions of the light spectra. Such flow cells are particularly suitable when combined with optical fibers for light transfer, enabling the design of a flexible sensor system. A number of flow cells having a long optical path length have been designed for absorbance, fluorescence, and Raman spectroscopy. Similar to optical fibers, light is confined in such flow cells within the (liquid) core by total internal reflection (TIR) at the liquid core/wall interface or the liquid core/cladding (coating) interface. The only requirement is that the liquid core refractive index be higher than that of the refractive index of the ambient. For liquid core comprising purified water, and for ambient being air, this requirement is easily satisfied.
One approach to employ liquid-based light guiding structures describes a reactor configuration for UV treatment of water utilizing TIR and a flow tube. The inlet and core of the cylindrical tank reactor unit is a transparent flow tube that is surrounded by a sealed, concentric volume of material having a lower refractive index than the fluid flowing in the flow tube, which enables TIR of UV light when it is directed axially into the flow tube. Another approach discloses a method and reactor for in-line treatment of fluids and gases by light radiation comprising a tube or a vessel made of transparent material, preferably quartz glass, and surrounded by air, and having a fluid inlet, a fluid outlet, and at least one opening or window adapted for the transmission of light from an external light source into the tube. Air outside the tube or vessel has a lower refractive index compared to the treated fluid, which enables TIR. Still other approaches discuss various aspects of a liquid core light guide. One such approach discusses a liquid core waveguide photon energy material processing system.
Other approaches discuss the fabrication of a liquid light guiding layer. FIGS. 1A and 1B show one approach for fabricating a fluoropolymer-based enclosure according to the prior art. In this approach, as shown in FIG. 1A, fluoropolymer pellets 2 are placed into a container 4. Subsequently, heat is applied, which results in the fluoropolymer pellets 2 melting into a continuous fluoropolymer-based enclosure 6 as shown in FIG. 1B. Such an approach is limited with respect to how thin and to what precision the dimensions of the enclosure 6 can be manufactured. Furthermore, selection of the fluoropolymer material forming the pellets 2 and the material of the container 4 needs to result in no significant adhesion between the fluoropolymer-based enclosure 6 and the container 4 to facilitate removal of the fluoropolymer-based enclosure 6 from the container 4.
FIGS. 2A and 2B show a side and top view, respectively, of a fluoropolymer-based enclosure 8 according to the prior art. In this case, a fluoropolymer-based film is folded over and subsequently fused along the edges 9A-9C. Such an approach requires a flexible fluoropolymer-based film and results in a flexible fluoropolymer-based enclosure 8, which may not be advantageous in some applications. Additionally, similar to other approaches, the exact dimensions, particularly the thickness, of the enclosure 8 are difficult to precisely control.